U.S. Pat. Nos. 6,868,289 and 7,016,725, each of which is incorporated herein by reference, disclose methods and apparatuses for treating tumors using AC electric fields in the range of 1-10V/cm, at frequencies between 50 kHz and 500 kHz, and that the effectiveness of those fields is increased when more than one field direction is used (e.g., when the field is switched between two or three directions that are oriented about 90° apart from each other). Those alternating electric fields are referred to herein as Tumor Treating Fields, or TTFields.